Surface Contamination of Stainless Steel Resulting in Red Rust Spots

A. Those rust spots are the result of surface contamination, often iron, although many other substances can cause a problem. It may be embedded into the surface so it can't be removed by simple cleaning.

Passivation should solve the problem by removing those contaminants.

First of two simultaneous responses --

A. Probably, low bid manufacturers used contaminated sand or glass bead and then failed to passivate the equipment. Improper maintenance (e.g., steel wool cleaning) or rusty tap water could also be responsible.

Suggest using a brush or non-abrasive scouring pad (white Scotch-Brite®) to apply a citric acid-based solution or gel to clean and passivate the surface. Stellar Solutions [a finishing.com supporting advertiser] is a supplier. Phosphoric acid cleaner/scale remover will also remove rust.

"The properties of stainless steel can change with continued use, especially under conditions where the chromium oxide layer is altered (e.g. incompatible cleaners, abrasive cleaners, abrasive cleaning pads, or chlorine and related sanitizers). Therefore, it is recommended that surfaces be passivated (using nitric acid or other strong oxidizing agents) initially and on a regular frequency thereafter, to maintain a passive (non-reactive) oxide film on the surface."
--- from Sanitary Design and Construction of Food Equipment, University of Florida (May 2005), http://edis.ifas.ufl.edu/FS119

Second of two simultaneous responses --

A. Dear Randall G. St.John,

Stainless steel do rust when it's surface is in active condition. Any metal when exposed to atmosphere produces its oxide layer at the surface. Rust is nothing but oxidation of Iron. Stainless Steel is a combination of Iron,Nickel Chromium and a few other elements.

To avoid rusting stainless steel surface has to be passivated. Passivation process makes the Stainless steel surface, Chromium rich, which produces Chromium Oxide that prevents further oxidation.

Conventional Nitric Acid passivation is a common one. You can get the info, if you go thru' metal finishing guide book. Or many other Proprietary chemicals also available, you can get many consultants who have their advertisements in this site itself..

regards,

A. There seem to be a lot of problems in the last few years with 304SS producing this problem. It is most likely inclusions in the steel, which can be enhanced by exposure to chlorides or other stressful environments. It is quite common to use chloride sterilization solutions in the food industry.
There are NSF certified citric formulations which can eliminate the rust spots and re-passivate the surface. Contact us if we can help.

A. Have the parts electropolished. It is a type of electrochemical passivation that is superior to standard passivation or pickling. It will completely remove all rust spots and other existing surface contamination and prevent further corrosion.
The resultant finish will benefit in the following ways:
-Super-Passivation
-Superior corrosion resistance
-Stress relieved surface
-Ultra-Hygienically clean surfaces
-Leaves a surface free of hydrogen and prevents contamination and multiplication of bacteria
-Decarbonized surface
-Reduces friction
-Brighter
-Smoother finish

A. Yes, electropolishing stainless steel 304 will eliminate all rusted areas and will provide the optimal anticorrosion protection. Portable spray electropolishing machines will be ready soon for electropolishing large construction fixtures or stainless tanks without submerging them into electrolyte.

A. Jalendu,
Electropolishing is great for corrosion resistance, always. Though at the very least, good citric acid based passivation products will also clear up that corrosion and get you back to a corrosion resistant surface, and this can be done safely and easily on-site.

Of course the ultimate best is electropolishing followed by passivation!